The overall objective of this research project is the understanding of neuronal circuitry within the hippocampal dentate gyrus. An electron microscopic analysis of Golgi-impregnated basket cells in the dentate gyrus will be combined with degeneration of entorhinal afferents in the same preparation. This study will determine if feed-forward inhibition occurs in this afferent pathway since basket cells provide a GABAergic, inhibition of granule cells in the dentate gyrus. This analysis will conclude the study of basket cell connections. New studies will concentrate on the hilar neurons of the dentate gyrus and determine if some of them give rise to inhibitory GABAergic commissural and associational projections. These studies will utilize a retrograde transport method in combination with immunocytochemical methods. The commissural projection has been considered to be exclusively excitatory and positive findings of double-labeled cells will provide evidence of a mixed inhibitory and excitatory pathway, a rare occurrence in the mammalian brain. If double-labeled neurons are observed, they will be described and analyzed in both light and electron microscopic preparations. In addition, Golgi-electron microscopic methods will be combined with retrograde techniques to determine if granule cells synapse with hilar commissural neurons. This series of experiments will provide morphological data on the connections of local circuit and projection neurons within the dentate gyrus of the hippocampus. These data will be helpful in the understanding of inhibitory mechanisms in the hippocampus.